Photoelectric conversion element having an infrared transmissive indium-tin oxide film

ABSTRACT

A photoelectric conversion device comprising a photoelectric conversion element having a photosensitivity to at least near infrared radiation and an indium oxide containing transparent film having a sufficient light transmission property to at least the near infrared radiation formed on the photoelectric conversion element is disclosed.

This is a continuation of application Ser. No. 08/271,627 filed Jul. 7,1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photoelectric conversion elementhaving an indium oxide-containing transparent conductive layer preparedby a coating and thermal decomposition method.

2. Prior Art

Thin metal films such as gold, silver, aluminum, and metal oxides suchas zinc oxide and ITO (Indium-Tin-Oxide) are generally known as atransparent conductive layer for use in solar cells. Sputtering methodsand vapor deposition methods are well known as methods for preparing theITO. (Electroceramics, '85. May, p. 23 (1985)). A method for preparingan indium oxide coating film by a coating and thermal decompositionmethod is described in Japanese Patent Publication No. Sho 54-28396.

However, the thin metal films such as gold, silver, and aluminum had lowlight transmission and the zinc oxide film having a high lighttransmission in the spectral range from the visible to the infrared hada low conductivity. In addition, the ITO transparent conductive layerprepared by a sputtering method or vapor deposition method had a highconductivity but the preparation process thereof was complicated and thelayer had no infrared transmission. Thus, there was no transparentconductive material for use in solar cells having a high conductivityand a high light transmission property in the range from visible toinfrared. Further, there was no disclosure of a combination of an indiumoxide film prepared by a coating and thermal decomposition method havinginfrared transmission and a high conductivity, and a photoconductivematerial having a photosensitivity to infrared rays.

SUMMARY OF THE INVENTION

Thus, the object of the present invention is to provide a photoelectricconversion device having a high light conversion efficiency by utilizingthe infrared energy of solar light effectively by combining atransparent conductive layer having a high light transmission in therange from visible to infrared and a high conductivity, with aphotoelectric conversion element layer comprising a photoconductivematerial having a photosensitivity to infrared rays.

The aforementioned object of the present invention can be attained bycombining an indium oxide containing transparent conductive layer formedby coating an organic indium compound containing solution and thermallydecomposing it, and a photoelectric conversion element layer comprisinga photoconductive material having a photosensitivity to infrared rays.

In accordance with the present invention, there is provided aphotoelectric conversion element having a sensitivity at to the nearinfrared region and an indium oxide containing transparent film having asufficient light transmission property to at least near infrared raysformed on the photoelectric conversion element.

The term "near infrared region" used in this application means lighthaving a wavelength in the range from 780 nm to 2000 nm.

The photoelectric conversion element of the present invention can beused in solar cells.

Referring to FIGS. 1(a) and 1(b), a solar cell utilizing thephotoelectric conversion element of the present invention may be formedby laminating a transparent conductive layer (2), a photoelectricconversion element layer (3), and an electrode (4) (conductive layer), aphotoelectric conversion element layer, and a transparent conductivelayer (2) successively on a substrate (1), or by forming a transparentconductive layer (2) on a photoconductive element (3) and an electrode(4) (conductive layer) on the opposite side (see FIG. 1(b)). Inaddition, the photoelectric conversion element layer may have alaminated structure comprising a junction between p type and n typelayers in order to generate photoelectromotive force. Further, the solarcell of the present invention may have another layer formed thereon. Forexample, a protective layer may be formed on the electrode (conductivelayer) or the transparent conductive layer.

Since the ITO transparent conductive layer of the present invention hasexcellent heat resistance, a photoelectric conversion element layer andanother layer can be formed on the ITO transparent conductive layer by aheating process, or a heat treatment can be performed after formingthese layers. Moreover, it can be used at a high temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) and (b) are schematic illustrations showing structures ofsolar cells.

FIG. 2 is a graph showing and comparing the changes in lighttransmission of ITO conductive films prepared by the coating and thermaldecomposition method of the present invention and by a conventionaldeposition method.

FIG. 3 is a graph showing and comparing the changes of electricalresistances of ITO conductive films prepared by the coating and thermaldecomposition method of the present invention and by a conventionaldeposition method.

DETAILED DESCRIPTION OF THE INVENTION

The transparent substrate used in the present invention may includequartz, non-alkali glass, and borosilicate glass. Each of these glasseshas transmission in the near infrared-ray region. The thickness of thetransparent substrate may not be limited. The organic indium compoundused for forming a transparent conductive film may include (R₁ COO)₃ In(wherein R₁ represents an alkyl group having 4 to 16 carbon atoms) and(CH₃ COCHCOCH₃)₃ In. In addition, in order to improve the conductivity,an organic tin compound may be added to the organic indium metalcompound containing solution. The organic tin compound may include (R₂COO)₂ Sn (wherein R₂ represents an alkyl group having 4 to 16 carbonatoms) and (CH₃ COCHCOCH₃)₂ Sn. The amount of the organic tin compoundadded is preferably 0 to 40 wt. %, more preferably 5 to 35 wt % by molarratio of tin/indium. If the molar ratio of tin is greater than the abovevalues, the conductivity tends to become low. The solvent used for theorganic indium compound containing solution may include most preferablyan aliphatic hydrocarbon such as octane, decane, dodecane, and tridecanefrom a view point of wettability of the transparent substrate such asquartz, non-alkali glass, and borosilicate glass. However, any solventhaving a solubility of the organic indium compound such as an aromatichydrocarbon such as toluene and xylene and a hydrocarbon halide such asmethylene chloride may be used. The solution concentration is preferablyin the range from 5 to 50%, more preferably 10 to 40% based on a solidscontent ratio. In addition, as an additive, a cellulose derivative suchas ethyl cellulose and nitro cellulose may be added to increase theadhesion to the substrate, or an unsaturated carboxylic acid such aslinoleic acid and linolenic acid may be added to improve the wettabilityof the substrate. The amounts added are preferably in the range from 0to 20%, respectively.

The coating method of the organic indium and other metal compoundcontaining solutions include bar coating, spin coating, spray coating,screen printing, and dip coating methods.

The method for thermally decomposing the coating film of the organicindium compound containing solution may be carried out in an electricoven, at a temperature in the range from 400° to 1500° C. for 30 min. to10 hours. If the calcining temperature is too low, the thermaldecomposition of the organic indium compound becomes insufficient andthe conductivity becomes low. If the calcining temperature is too high,the component(s) of the substrate will penetrate into the indium oxidefilm and the conductivity becomes low. The calcining time may be equalto or greater than 10 hours, but this is not particularly required.Calcination may be carried out in an air, a nitrogen, or an oxygen flow,a nitrogen or oxygen substituted atmosphere, and a reduced pressuredatmosphere. The film thickness of the calcined oxide is preferably inthe range from 0.05 to 10 μm, more preferably 0.1 to 5 μm. If thethickness is less than these values, the conductivity tends to becomelower and if the thickness is greater than these values, thetransparency tends to become reduced. In addition, the coating andthermal decomposition of the organic indium compound containing solutionmay be repeated in order to increase the film thickness.

The photoconductive material contained in the photoelectric conversionelement layer may include an inorganic elemental or compoundsemiconductor such as silicon (in amorphous, single crystalline, orpolycrystalline form), GaAs, CdS, and an organic semiconductor compoundsuch as squarylium and phthalocyanine compounds. A photoconductivematerial having an absorption of infrared rays equal to or greater than780 nm is usable. In addition, an additive such as boron or phosphorusmay be added in order to provide p-type and/or n-type photoconductivity.The photoelectric conversion element layer may include a p-n junction, aSchottky barrier, and a BSF, but it is not limited to these types. Theprocedures for making the photoelectric conversion element layer includeforming a photoelectric conversion element layer on a transparentconductive film formed on a substrate such as glass, and forming atransparent conductive film on a photoelectric conversion element layer.Further, the method for preparation may be selected from vapordeposition, plasma CVD, a casting, a CZ, a coating method, or dispersionin a resin, depending on the materials or structures of thephotoelectric conversion element layer. However, the method is notlimited to these. Further, since the transparent conductive film of thepresent invention has excellent heat resistance, the formation of aphotoelectric conversion element layer requiring a heat treatment suchas annealing or calcining a substrate on which a transparent conductivefilm is formed, or a heat treatment after the formation of thetransparent conductive film and the photoelectric conversion elementlayer are possible.

The conductive layer used as a backside electrode includes metals suchas gold, silver, platinum, and aluminum.

Since the transparent conductive film formed by coating and decomposingthe organic indium compound containing solution has a high transmissionof infrared rays, a solar cell in which it is combined with aphotoelectric conversion element layer comprising a photoconductivematerial having a photosensitivity to infrared rays can have improvedconversion efficiency because it has lower infrared ray loss. Inaddition, it acts as a reflection prevention film because it has a lowreflection rate in the range from the visible to the infrared.

EMBODIMENT

The present invention will be hereunder described with examples andcomparative examples.

EXAMPLE 1

A p-type polycrystalline silicon substrate doped with boron having a 10cm² area and a 0.4 mm thickness and made by a casting method was dopedwith phosphorus on one side to form a 1 μm deep n-type silicon layer. Asolution of 0.5 g of (C₈ H₁₇ COO)₃ In and 0.013 g of (C_(s) H₁₇ COO)₂ Sndissolved in 1.857 g of dodecane was coated on the n-type silicon layerwith a wire bar (#10), dried at 45° C. for 30 min., calcined at 800° C.for one hour in air to form a transparent conductive film of In₂ O₃/SnO₂) having a thickness of 0.2 μm on the polycrystal siliconsubstrate. Further, aluminum was deposited on the other p-type siliconsurface of the polycrystalline silicon substrate to form a backsideelectrode layer having a film thickness of 1 μm. The conversionefficiency of the solar cell thus prepared was 7.8%.

EXAMPLE 2

A solution of 0.5 g of (C₈ H₁₇ COO)₃ In and 0.013 g of (C₈ H₁₇ COO)₂ Sndissolved in 1.857 g of dodecane was coated on a quartz substrate with awire bar (#10), dried at 45° C. for 30 min., calcined at 800° C. for onehour in an air to form a transparent conductive film of In₂ O₃ /SnO₂having a thickness of 0.2 μm. On the film, a perylene compound layerhaving a thickness of 0.1 μm and a hydroxy squarylium compound layerhaving a film thickness of 0.1 μm were successively deposited to form aphotoconductive conversion element layer. The structural formulas ofperylene compound (I) and hydroxy squarylium compound (II) are shownbelow: ##STR1##

Further, gold was deposited on the organic compound layer to form anopposed electrode layer having a film thickness of 1 μm. The conversionefficiency of the solar cell thus prepared was 0.4%.

COMPARATIVE EXAMPLE 1

A solar cell was prepared using the method as described in Example 1,except that an ITO transparent conductive film having a film thicknessof 0.2 μm was prepared by vacuum deposition. The conversion efficiencyof the solar cell thus prepared was 6.8%.

COMPARATIVE EXAMPLE 2

A solar cell containing the hydroxy squarylium compound was preparedusing the same method as described in Example 1, except that an ITOtransparent conductive film having a film thickness of 0.2 μm was madeby vacuum deposition on a quartz substrate. The conversion efficiency ofthe solar cell thus prepared was 0.3%.

Comparing the ITO transparent conductive film prepared by vacuumdeposition (Comparative Example) and the ITO transparent conductiveprepared by the coating and thermal decomposition method (Example), theITO transparent conductive film formed by coating and thermaldecomposition method shows a high light transmission in the range fromthe visible to the infrared. On the other hand, the light transmissionof the ITO transparent conductive film formed by vacuum depositiondecreases from about 700 nm as shown in FIG. 2. Since there aredifferences in the light transmission at 700 nm and above between theITO transparent conductive film made by vacuum deposition and the ITOtransparent conductive film made by the coating and thermaldecomposition method, the solar cell having the ITO transparentconductive film prepared by the coating and thermal decomposition methodhas a higher conversion efficiency than other solar cells usingphotoconductive materials having absorption in the infrared region.Besides the ITO transparent conductive film prepared by vacuumdeposition, an ITO transparent conductive film prepared by a sputteringmethod also showed a decrease in light transmission from above 700 nmsimilar to the one prepared by vacuum deposition.

Comparing the heat resistances, the electrical resistance values of theITO transparent conductive film by the coating and thermal decompositionwere unchanged by thermal treatment, while on the other hand, theelectrical resistance values of the ITO transparent conductive filmformed by vacuum deposition increased as shown in FIG. 3. Thus theformation of a photoelectric conversion element layer by heat treatmentof the ITO transparent conductive layer made by the coating and thermaldecomposition method and by heat treatment after the formation of thetransparent conductive film and the photoelectric conversion elementlayer can be employed. Therefore, the ITO transparent conductive filmformed by the coating and decomposition method of the present inventioncan be applied to materials for windows used at high temperatures suchas in electric ovens and to materials used for windows of airplanes andautomobiles requiring electrical conductivity.

As described above, the present invention has the effect of increasingthe photoconversion of a solar cell by combining a transparentconductive layer having a high light transmission in the range from thevisible to the infrared and a high conductivity, with a photoconductiveconversion element layer containing a photoconductive material having aphotosensitivity to infrared rays.

In addition, by preparing a transparent conductive layer by the coatingand thermal decomposition method of the present invention, the area canbe easily increased without using complicated devices for sputtering orvapor deposition. Further, a transparent conductive layer may beprepared with a simple process so as to form a predetermined pattern,such as by coating and screen printing without an etching process, andthus the preparation process of a solar cell may be simplified. Inaddition, the solar cell of the present invention may be applied topower sources of various apparatuses and generators for electricitygeneration using clean solar energy.

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. A photoelectric conversion device comprising aquartz substrate showing transparency to at least the near infraredregion, an indium-tin-oxide containing transparent electrode; whereinsaid indium-tin-oxide containing transparent electrode is formed from asolution comprising an organic indium compound and an organic tincompound dissolved in an aliphatic hydrocarbon solvent, wherein saidorganic tin compound is chosen from (R₂ COO)₂ Sn, wherein R₂ is an alkylgroup having 4-16 carbon atoms, or (CH₃ COCHCOCH₃)₂ Sn; and wherein saidtransparent electrode has 90% or more of light transmission to at leastlight having a wavelength in the range from 500 nm to 2,000 nm; aphotoelectric conversion element having a photosensitivity to at leastthe near infrared region, and a second electrode, wherein saidtransparent electrode, said photoelectric conversion element and saidsecond electrode are successively formed on said quartz substrate.
 2. Aphotoelectric conversion device as claimed in claim 1, wherein theindium-tin-oxide transparent electrode is formed by coating saidsolution on the photoelectric conversion element or the quartz substrateand then thermally decomposing it.
 3. A photoelectric conversion devicecomprising a quartz substrate showing transparency to at least the nearinfrared region, an indium-tin-oxide containing transparent electrode;wherein said indium-tin-oxide containing transparent electrode is formedfrom a solution comprising an organic indium compound and an organic tincompound dissolved in an aliphatic hydrocarbon solvent, wherein saidorganic tin compound is chosen from (R₂ COO)₂ Sn, wherein R₂ is an alkylgroup having 4-16 carbon atoms, or (CH₃ COCHCOCH₃)₂ Sn; and wherein saidtransparent electrode has 90% or more of light transmission to at leastlight having a wavelength in the range from 500 nm to 2,000 nm; aphotoelectric conversion element having a photosensitivity to at leastthe near infrared region, and a second electrode, wherein said secondelectrode, said photoelectric conversion element, and said transparentelectrode are successively formed on said quartz substrate.
 4. Aphotoelectric conversion device consisting essentially of anindium-tin-oxide containing transparent electrode; wherein saidindium-tin-oxide containing transparent electrode is formed from asolution comprising an organic indium compound and an organic tincompound dissolved in an aliphatic hydrocarbon solvent, wherein saidorganic tin compound is chosen from (R₂ COO)₂ Sn, wherein R₂ is an alkylgroup having 4-16 carbon atoms, or (CH₃ COCHCOCH₃)₂ Sn; and wherein saidtransparent electrode has 90% or more of light transmission to at leastlight having a wavelength in the range from 500 nm to 2,000 nm; aphotoelectric conversion element having a photosensitivity to at leastthe near infrared region, and a second electrode, wherein saidtransparent electrode is formed on one side of said photoelectricconversion element and said second electrode is formed on the oppositeside of said photoelectric conversion element.